During the last half-century we have been so saturated with wonderful discoveries and inventions that we can form but a faint idea of the sensation produced in the minds of the people of the seventeenth century by the discovery of the telescope. Great discoveries, such as have crowded upon us so rapidly since the invention of the electric telegraph, were then few, and separated by centuries instead of by months as now.

How great must have been the wonder excited by this seeming reversal of the laws of nature, the apparent annihilation of distance by this marvelous instrument, most of our readers can probably have but a faint idea.

The writer has some realization of it, his surprise, wonder and delight at his first clear sight through a spyglass, at the age of nine, being still fresh in his memory.

Galileo has commonly had the credit of the discovery. But the idea was suggested previously by Kepler, and it is very probable that the Italian's invention was at least stimulated by seeing mention of the general principle, possibly in a letter from Kepler, with whom he corresponded.

As it happens, however, his invention was paralleled, if not anticipated, by two spectacle-makers in Middel-burg, Holland, Zachariah Jansen and Hans Lipperheim, who independently made the same invention, except for the use of a convex instead of a concave eyeglass. But Jansen and Lipperheim are not such picturesque historical figures as Galileo, and so their fame has not lived as has the latter's.

The two forms are optically different, Galileo's, with the concave eyeglass e, as shown in the diagram, Fig. 1, intercepting the rays from the objective o, before they come to the focus, and making them parallel by bending them out, so that the only real image is formed within the eye.

Jansen's and Lipperheim's, on the other hand, using the convex eyeglass, placed beyond the focus of the objective at a distance equal to its own focal length, magnifies the image formed at that focus, rendering the rays parallel by bending them inward. (Fig. 2.)

Fig. 3.

Fig. 1

Both these forms, in their essentials, survive in modern practice, Galileo's in the opera and field glasses, and Lipperheim's in the modern refractor, whose furthest point of growth is now the Yerkes equatorial.

Fig. 2.

The Galilean telescope had the defect, inherent in its construction, of possessing a very small field of view, which more than offset its sharper definition, so that it was soon dropped in astronomical practice, and the Dutch arrangement became practically the only one in use.

It was soon found that the prismatic effect of the lenses limited the possible aperture of the objective and power of the eyeglass, so that to obtain, for instance, a power of fifty, it was necessary to have a telescope six feet long, whose objective was limited to 1.32" in diameter; astronomers mentioned the length of the telescope employed in any given observation, as indicative of the power used, and this practice continued even as late as the middle of the nineteenth century, although its force as indicating power was entirely obsolete.

So the telescopes lengthened out in search of power, until the devices for handling the tube and keeping it from bending under its own weight became so heavy and unwieldy that observing with them was almost impossible.

Huyghens, who, besides being an astronomer, was, for those days, an excellent mechanical engineer, invented an arrangement for using the telescope without a tube. The objective, set in a short tube, mounted on a ball-and- socket joint, was arranged so as to be raised and lowered on a tall pole; the axis of the short tube was controlled by a long cord, so that another short tube, carrying the eyeglass and held in the hand, could be brought into line with it. The illustration, Fig. 3, shows the appearance of one of these telescopes. aa is the post, b the bracket carrying the objective, which is mounted on the ball-and-socket joint c, and balanced by the weight g. The bracket and its load are counterpoised by the weight f, which is carried on an endless cord passing over the pulley d. The observer leaned on the rest h, and controlled the objective and kept it in line with the eye-tube e, by the cord i. The image of the object was received on the cardboard disc shown at the front end of the eye-tube, and the latter directed into place by an assistant.

With an instrument of this kind Huyghens discovered the fourth satellite of Saturn, and determined the form of his ring, which was before unknown; and Cassini discovered three other satellites of the same planet, and made his other discoveries, with such telescopes.

With such an equipment, observing must have been a terrible labor, and all these discoveries were made at such cost of toil and exposure as would daunt most modern observers.

These telescopes were made of dimensions up to 150 feet in length, and Auzout, of Paris, is said to have made an object-glass of 600 feet in focal length, but there is no record of its ever having been mounted for any useful observations.

The difficulties of observing with such glasses set the wits of many scientific men to work to find a remedy. Sir Isaac Newton and others attempted to solve the problem of making an object-glass free from color, but all failed, the difference in the refractive qualities of different kinds of glass being then unknown. Newton then turned his attention to the use of mirrors, and in 1072, with his own hands, made two small reflecting telescopes; these were about six inches in focal length, and one of them bore a power of 38, about equal to that of a four-foot refractor of that time. The performance of these instruments left something to be desired, as Newton, though, as he says, he approved the parabolic figure, knew no way of producing it, and had to content himself with spherical curves, which do not produce a perfect image. One or both of these telescopes is still in existence in the collection of the Royal Astronomical Society, if I am not mistaken.